Those skilled in the art are familiar, in general, with brake pressure boosters of this kind (see, e.g. DE-GM 92 02 154.9).
In known brake pressure boosters making use of vacuum, the low pressure chamber normally is evacuated to a value of e.g. 0.2 bar, while pressures between 0.2 and 1 bar are adjusted in the high pressure chamber, depending upon the boosting effect desired. The boost obtainable when using a conventional brake pressure booster of this kind thus corresponds to a pressure difference of approximately 0.8 bar.
Often not only a vacuum source is available in motor vehicles. Likewise to be made available in simple manner is a source of air having a pressure which is higher than that of the external atmosphere (hereinafter referred briefly as "atmospheric pressure").
A brake pressure booster including the features recited in the precharacterizing part of claim 1 is known from JP 59-134 047 A (Patents Abstracts of Japan, Section M, vol. 8, 1984, no. 262 (11-341). In that case, however, the switchover of operation of the brake pressure booster is effected under conditions at which pressure is fed into the high pressure chamber of the brake pressure booster, in which chamber the pressure is higher than atmospheric pressure, with losses occurring in respect of the pressurized air. In other words, air from the pressure source gets lost upon switchover. This loss of compressed air is unavoidable in the prior art device cited, which is known from JP 59-1343 047, because a slide valve is provided which must adopt an intermediate position upon switchover, for reasons of safety and functioning. And while in this intermediate position, the air pressure source and the inlet of the valve leading to the outside atmosphere are interconnected temporarily.
Also EP 0 368 691 A1 describes a brake pressure booster with which a pressure source is used to feed air of higher pressure than the outside atmospheric pressure into the booster. With this prior art, however, the supply of pressurized air is controlled entirely separately, i.e. irrespective of the feeding of air into the high pressure chamber of the brake pressure booster at pressures below atmospheric. This prior art furthermore is disadvantageous because the compressed air pressurizes only a limited ring section (bellows) so that the resulting brake pressure booster on the whole becomes quite large.